1. Field of the Invention
The present invention relates to electrical power distribution systems, and more particularly to filters for mitigating harmonic currents in such systems.
2. Description of the Related Art
Electrical loads are commonly controlled by power converters, such as adjustable speed drives for operating large electric motors, which convert AC power at one frequency to another frequency. Power converters for electric motors often are bidirectional so that when the motor is being driven by the mechanical load in a regenerative mode, the motor acts as a generator sending electrical current back through the power converter into the utility lines.
Non-linear electrical loads produce various levels of harmonic currents in the power distribution system with the harmonic frequency spectrum depending upon the nature of the load. The harmonic currents create many problems in the power distribution system, including increased total harmonic distortion voltage level, reduced power distribution equipment reliability, reduced electromagnetic compatibility of the loads, increased power losses, reduced power factor, and other problems which are well known to those skilled in the art.
Many apparatus have been proposed for mitigating harmonic currents. For example, filters were placed at the utility line connections to reduce the transmission of harmonics from the utility lines to the electrical load in the motoring mode and from the electrical load to the utility lines in the regenerative mode. However, conventional filters added an undesirable amount of reactance between the supply lines and the load.